In cardiac and thoracic PET or PET/CT, the image reconstruction procedure requires a good estimate of the attenuation suffered by pairs of emitted photons. In measuring this and converting the measurements to correction factors, it may be particularly important to account for attenuation edges related to the boundaries between lung and soft tissue, including the moving diaphragm and heart wall. It is axiomatic that one may need to correct the PET data with a different pattern of attenuation correction factors at each moment in time, yet this is not done in normal practice. Instead, one makes do with approximate solutions.
Conventional PET scanners make a transmission measurement lasting several minutes, based on a line source orbiting between the patient and the PET scanner's detectors. In that case, one measures the time-averaged attenuation due to the patient as his chest may have moved through many configurations. Arguably, this may result in a good standard for cardiac PET. In the case of PET/CT, attenuation maps are derived either from the “snapshot” provided by a fast CT scan (to use a metaphor from ordinary photography) or from the “time-exposure” obtained in a slow CT scan. Fast CT scanning of the chest fails to represent faithfully the configuration of attenuating tissue, which is in constant motion. Slow CT somewhat averages organ motions in the chest, but the averaging is not identical to the motion that occurs in a PET scan, which typically lasts for many minutes. Still another approach is the time-averaged CT scan. Of these various approaches, the fast static CT has the advantage of a lower radiation burden.
A need exists to measure the attenuation more accurately to give a better calculation of an emission image. This may improve the quality of the image reconstruction procedure and the resulting image.
Besides the continuous quest for improved image quality, it may be desirable to have an efficient and/or sensitive medical device performing PET, PET/CT, SPECT or SPECT/CT. This would allow for a reduction in time for taking images, an improved quality of the images, and/or a reduction of exposure of a subject to the image apparatus.
Additionally, it is desirable to avoid cumbersome and time consuming arrangements or methods, in an economic and technical perspective, for measuring the attenuation accurately.